Electric water heaters that can heat water at any time of day are by far the most expensive option. Off-peak electric (which heat only when tariffs are lower) and gas systems have similar costs, depending on the tariffs you have to pay.

Natural gas produces much less of the greenhouse gas carbon dioxide (CO2) than electricity to heat the same amount of water. But it’s not available everywhere.

There are two main types of water heater: storage and instantaneous. Instantaneous heaters heat the water instantly, while in storage heaters it’s stored in a tank.